1. Field of Invention
This invention pertains to the recording of information on magnetic tape using magnetic tape drives, and particularly to apparatus and method for performing backhitch operations in helical scan magnetic tape recording.
2. Related Art and Other Considerations
For decades magnetic tape has been employed as a medium for storing information. Devices known as tape drives, tape decks, or tape recorders perform input and output operations, e.g., reading and recording operations, by transducing information from and to the tape. Information to be stored on a tape is obtained from a host device such as a computer. The information is transmitted from the host over a special connection or bus, e.g., SCSI bus, to the tape drive. Internally the tape drive has a buffer memory for storing information obtained from the host which the drive is not quite ready to record on the tape. When the information is ready to be recorded, the information passes through a write channel of the tape drive to a head. The head has gaps or other appropriate elements thereon which form magnetic flux transitions on the tape in the recording operation.
A reading operation for a tape drive is essentially the reverse of the recording operation. In the reading operation, the head detects magnetic flux transitions on the tape to obtain a read signal, processes the read signal in read circuitry, stores information ascertained from the read circuitry in the buffer, and ultimately transmits the information to a utilization device, e.g., the host, over the bus which connects the host and the tape drive.
One type of magnetic recording is helical scan recording. In helical scan recording, one or more head(s) are mounted on a rotating drum. The tape is transported past a portion of the periphery of the rotating drum so that, as the head on the drum contacts the tape, a stripe or track is recorded on the tape at an angle to the direction in which the tape is transported. The tape is transported through a tape path, which includes around the periphery of the drum, from a tape supply reel to a tape take-up reel. In some magnetic tape drives, a capstan is utilized to impart linear velocity to the tape. By contrast, a capstanless helical scan tape drive is shown in U.S. Pat. No. 5,602,694 for CAPSTANLESS HELICAL DRIVE SYSTEM to Robert J. Miles and James Zweighaft, which is incorporated herein by reference.
Helical scan tape drives record information on tape, and read information from tape, at fixed data rates. Frequently the tape motion must be stopped during normal data transfer. In the case of recording on tape, the tape drive must stop the tape motion when the data buffer is near empty, so that it can wait for the host to send more data to the tape drive. In the case of reading from tape, the inverse is true, in that the tape drive must stop tape motion after the buffer has been filled so as to wait for the host to empty the buffer. Stopping of tape motion also occurs after any operation is completed by the tape drive before the next command is issued from the host.
When tape motion is stopped, the last data transduced relative to the tape occurs at a data stop point before the tape decelerates to a stop. After the tape is stopped, the next command from the host to the tape drive will likely result in a continuation of the same type of data transfer that had occurred just prior to the stopping of the tape motion. Resumption of the data transfer requires, however, that the tape be accelerated back up to a proper or nominal tape transport rate for the data transfer, and that the head be located at the data stop point prior to forming a write splice upon further data transfer. Traditionally a motion known as a backhitch was implemented in order to enable such a resumption or write splice.
In a conventional backhitch, in order to resume data transfer, the tape had first to be moved in a reverse direction to position the tape before the data stop point, e.g., at a point earlier involved in the data transfer operation. Then, from that earlier point, the tape is accelerated in order to be up to nominal tape transport speed at the time the head reached the stop point. FIG. 6 graphically shows a conventional backhitch, with a tape traveling at a velocity V.sub.F in the forward direction. Point P.sub.DS shows a data stop position whereat the last data is transduced prior to deceleration of the tape. Point P.sub.TS shows the actual stop point of the tape. When data transfer is to resume, the tape must be accelerated to a rewind velocity V.sub.R, then decelerated to a second tape stop point P.sub.TS2 (the "earlier" point). Thereafter the tape is accelerated in the forward direction to reach velocity V.sub.F so that transducing can occur with the tape at nominal velocity when the head(s) pass point P.sub.DS. The backhitch of FIG. 6 thus has a football type graphical illustration.
A conventional backhitch typically takes about 1.5 seconds. From the standpoint of data transfer, this backhitch time is a total loss, since the tape drive cannot record or read during the backhitch repositioning. Since the backhitch repositioning is a loss, the data buffer must be sized sufficiently to compensate for or hide this lapse in data transfer to/from the host. For example a tape drive in a record mode may have a data rate to tape of 4 MegaBytes per second (MB/s). If the host were providing data to such drive at a rate slightly less than the data rate to tape, e.g., 2.5 MB/s, the drive would eventually deplete the buffer and be forced to stop. The host would then be filling the buffer for 1.5 seconds while the drive was repositioning for sake of the backhitch. In order to avoid stopping the host, the drive's buffer would need to be 1.5.times.2.5=3.75 MB to cover the backhitch. Thus, the conventional backhitch influences the size of memory required for the buffer.
What is needed therefore, and an object of the present invention, is a method and apparatus for efficiently positioning tape in context of a backhitch.